A single dose of the Covid-19 vaccine made by Johnson & Johnson is highly effective in preventing serious illness and death from Delta and Beta variants of the coronavirus, data from a clinical trial in South Africa suggests.

The study is the first real-world test of the efficacy of the vaccine against Delta, a highly contagious variant of the virus that is sweeping the United States and much of the world. The South African Department of Health reported on these preliminary findings at a press conference on Friday. The data has not yet been peer reviewed or published in a scientific journal.

In the trial, called Sisonke, researchers evaluated a dose of the Johnson & Johnson vaccine in nearly 500,000 healthcare workers, who are at high risk of Covid-19. The vaccine is up to 95% effective against death from the Delta variant, and up to 71% against hospitalization, the researchers reported. (The vaccine performed slightly worse against the beta variant, which is believed to be more able to bypass the immune response than Delta.)

“We believe this vaccine does what it was designed to do, which is to prevent people from going to the hospital and to prevent them from ending up in intensive care and dying,” said Dr Linda-Gail Bekker, study co-leader and director of the Desmond Tutu HIV Center at the University of Cape Town.

The results suggest that people who received a dose of the Johnson & Johnson vaccine do not need a booster, Dr Bekker said.

When so-called “breakthrough” infections occurred in vaccinated volunteers, they produced mild symptoms in 96 percent of cases and resulted in serious illness or death in less than 0.05 percent, according to the study. The trial took place from February to May this year.

The results should give comfort to the millions of people who have received the Johnson & Johnson vaccine, especially because some previous studies had suggested that the single vaccine may be vulnerable to Delta. Laboratory studies on the vaccine’s performance against the variant have been mixed, and a trial comparing the one- and two-dose regimens has yet to publish results.

Given the uncertainty, some Johnson & Johnson beneficiaries have requested a second dose themselves. San Francisco Health Authorities Offer Residents Immunized With Johnson & Johnson Supplement dose of Pfizer-BioNTech or Moderna vaccines.

South Africa approved the Johnson & Johnson vaccine in April. The country battled a wave of the beta variant of the virus earlier this year, but Delta has become the dominant variant in recent weeks. More than 8 million South Africans have received the Johnson & Johnson vaccine or at least one dose of the Pfizer vaccine.

“This study was conducted as a real-world efficacy study in one of the most challenging epidemiological settings,” said Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston who has led some studies for J. & J. but did not participate in the trial. “This is very good news for the fight against the global Covid-19 pandemic. “

South African researchers recorded two cases of a rare bleeding disorder associated with the Johnson & Johnson vaccine in the trial; both participants made a full recovery.